Upon use, a sensor must be fixed in a position where it can detect a detection subject accurately. Therefore, in JP2001-271918A, a rotation sensor that detects a rotation speed of a rotational element in an automatic transmission in order to control the automatic transmission is attached via a bracket to a control valve provided on a case lower portion of the automatic transmission. More specifically, an oil pressure sensor and an oil temperature sensor are attached to the bracket together with the rotation sensor to form a sub-assembly, whereupon the sub-assembly is fastened to the control valve by a bolt.